An accepted way to control the attitude of a vehicle in space is the use of a momentum wheel. Typically this comprises a balanced body mounted for rotation about an axis in the vehicle, and having means for angularly accelerating or decelerating the body about the axis. By the general principles of momentum exchange theory, angular acceleration of the body about the axis in one direction results in angular acceleration of the vehicle about the axis in the opposite direction. Since the mass of the body is much less than the mass of the vehicle, a considerable acceleration of the body is required to produce a significant acceleration of the vehicle, but a considerable change in vehicle attitude about the axis can be produced, although quite slowly, by accelerating the body and then maintaining it in operation at the accelerated rate.
As long as the body continues to turn at a constant angular velocity, including but not limited to zero, the vehicle itself continues in its then angular velocity. When the vehicle has assumed a desired new attitude, deceleration of the body to zero results in deceleration of the vehicle to zero, and the newly achieved attitude is maintained.
In the real world, there are external torques due to the slight drag in the "vacuum" of space, as well as to the "light pressure" from the sun. There may also be rotating machinery aboard which accelerates or decelerates to produce unwanted torques of the vehicle. Over a period of time, to counter these torques, the momentum wheel will not typically be at zero angular speed for a space vehicle which is at zero angular speed, and means are accordingly provided for "dumping" angular momentum, by torquing magnetically on the Earth's magnetic field, for example.
It is customary to mount a momentum wheel with its axis of rotation aligned with one of the major axes of the vehicle, arbitrarily defined as X-, Y-, and Z-axes. By this means it is possible to bring the vehicle to any desired position about the axis of the momentum wheel. If it is desired to be able to bring the vehicle to any desired attitude in space, it is possible to provide multiple momentum wheels, rotatable about additional axes which may or may not be perpendicular to each other and to the first axis. By acceleration and deceleration of the wheels in sequence, it is possible to bring the vehicle to its desired position in steps, which however might be a somewhat protracted procedure.
In practice it is desirable to accelerate more than one wheel at the same time, to reduce the length of time required to bring the vehicle to a desired position, and for continuous attitude control this is essential. This, however, introduces a disturbing factor. A spinning momentum wheel acts as a gyroscope, and when it is attempted to give to a vehicle, having a first wheel spinning about a first axis, an acceleration about a second axis, by use of a second momentum wheel, the gyroscopic torques produced by the two wheels interact in a way known as cross coupling between the axes, compensation for which is required, but is generally not complete.
Another characteristic of momentum wheels rotating about multiple axes is that perfect balance with conventional support means, i.e., bearings, is not achieved, and the rotation of the unbalanced momentum wheels induces vibration disturbances into the vehicle body. Also conventional bearings have wear and life problems which limit the useful lifetimes of the vehicles in which they are installed.
Significant efforts have been made to isolate the momentum wheel vibration disturbances and increase the life of the single axis momentum wheels by using magnetically supported wheel bearings. This requires radial support in two directions and axial support in a third which is not symmetrical. Rotation of the spacecraft in which such a single axis magnetically supported momentum wheel is rotating results in significant gyroscopic loads on the magnetic support bearings. So far a practical single axis magnetically supported momentum wheel has not been produced.